This invention is concerned with television systems, and is particularly directed to projection television systems in which discrete images are projected on a projection screen to provide a composite color picture.
FIG. 1 is a schematic plan view of the essentials of a representative prior art projection television system 6 in which a bank 8 of light projection devices 10, 12 and 14 project through the indicated associated lens means blue, green and red images, respectively, on a remotely located projection screen 16. The projected images are intended to form the composite color image. Projection screen 16, which may be either of the front-projection or rear-projection type, typically has an aspect ratio of 3:4 in consonance with the aspect ratio of the standard television picture.
The centrally located device 12 typically has its projection optical axis 18 in congruence with screen axis 20, and as a result projects an undistorted light image on projection screen 16. The light images projected by devices 10 and 14, however, whose projection optical axis 22 and 24 respectively are located "off-axis" with respect to projection screen axis 20, inherently project light images which are distorted because of their off-axis location.
Two types of optical distortion are inherent in the system which can degrade through misconvergence the composite projected image to the point of unacceptability. The types are trapezoidal distortion and horizontal non-linearity, and they can best be described by the single term "non-linear magnification distortion". As noted, the light image projected by the centrally located device 12 is not subject to non-linear magnification distortion because its projection optical axis 18 is congruent with the projection screen axis 20. As a result, the light image projected on projection screen 16 will by symmetrical and undistorted. The light image projected by device 12 is typically that shown in FIG. 2 by light image 26, indicated as being rectilinear.
This is not so with the light images as projected by devices 10 and 14. The inherent distortion of the light images due to the off-axis location is depicted by FIG. 2, wherein the light image projected by device 10 is indicated by configuration 28 as being trapezoidal. Similarly, the image projected by device 14 is indicated by configuration 30 as being trapezoidal. In a typical prior art projection television system, the non-linear magnification distortion may be of the order of five percent, an amount sufficient to so misregister the images as to render the composite picture unacceptable to the viewer.
It is to be noted that if the projection devices 10, 12 and 14 are vertically stacked, a similar distortion will be realized in the case of devices 10 and 14. The non-linear magnification distortion realized is described in the context of this disclosure, as "keystoning" distortion, rather than "trapezoidal" distortion.
The second form of distortion--horizontal non-linearity--is also in consequence of the location of devices 10 and 14 off the projection screen axis 20. The effect of this type of distortion is depicted in FIG. 3, using as an example the light image 28 projected by light projection device 10, indicated as being trapezoidal due to the aforedescribed non-linear magnification distortion (the trapezoidal shape is exaggerated for the purpose of illustration). The lines 32A-G represent the vertical lines of a television screen cross-hatch generator, as projected. The effect of horizontal non-linearity distortion is apparent in the progressive increase from left to right in the distance between lines 32A-G. The vertical lines of the light image 30 projected by device 14 would be similarly distorted, but in the opposite direction.
One approach to the correction of trapezoidal distortion is by electronic means. For example, the image projected by the off-axis cathode ray tubes of projection means 10 and 14 can be made compensatorily trapezoidal. This can be done by synthesizing a correction wave form for application to a high-speed writing-type yoke which is placed in tandem with the main deflection yoke. The end result is a trapezoidally shaped raster inverse in orientation to the normal distortion of the image projected by the off-axis cathode ray tubes of projection means 10 and 14. Correction by such electronic means is plagued by the complications introduced in the television circuit, with a consequent increase in cost. The complexity and added cost is even greater in consequence of the fact that the correction circuitry for the two off-axis CRT's must be designed to exert an opposite effect on their projected images. The economic burden imposed by the electronic approach is further underscored in view of the fact that while it may be effective against trapezoidal distortion, it is largely ineffective in terms of correction for horizontal non-linearity distortion, wherein additional and very complex electronic correction circuitry must be employed.
Optical systems for reducing or otherwise ameliorating distortion include Oland--U.S. Pat. No. 4,004,093, which discloses a truncated Schmidt optical system wherein a plurality of Schmidt systems are clustered closely together by truncating the mutually adjacent edges of mirrors and correcting lenses which comprise individual Schmidt systems. Such clustering is said to provide a reduction in trapezoidal distortion by virtue of the fact that the cathode ray tubes for each primary color project images which arrive almost orthogonally at the screen.
Hergenrother et al--U.S. Pat. No. 4,024,579--discloses a projection television system in which the composite image is projected onto a curved screen by three discrete cathode ray tubes arranged as a triad, with each projecting a different primary color. The tube optics are folded into a catadioptric configuration and the three images are caused to converge into a composite image by an elaborate optical system that includes a Schmidt correction lens mounted externally to the envelope of each tube. Although the system has achieved a measure of consumer acceptance, the need for an extensive alignment procedure to achieve a satisfactory composite image, and the general lack of brilliance of the projected image, has limited its acceptance.
It is known in the art that if the axis of an electron gun is at an angle with respect to the axis of a cathode ray tube, the visible image on the face panel will exhibit non-linear magnification distortion. This type of distortion was common to certain early image iconoscope tubes of Vladimir Zworykin and was considered a performance liability. An example of a cathode ray tube configuration having an electron gun at an angle with respect to the tube axis is shown in U.S. Pat. No. 2,777,084 to Lafferty.
Pat. No. 28 37 249 (German) discloses a system for optically correcting trapezoidal distortion of the image projected by cathode ray tubes located off a central axis. The projection system includes cathode ray tube color picture sources, each of which projects its image through a projection lens. Trapezoidal distortion is stated as being corrected by tilting the picture sources away from the central axis relative to the light axis of the associated projection lens. As a result, the image on the faceplate of the cathode ray tube is no longer parallel with the screen. When the image on the cathode ray tube faceplate is projected, the image on the screen is reputed to be parallel.
Ohmori, in U.S. Pat. No. 4,194,216, discloses a video projection apparatus having a plurality of cathode ray tubes and associated projection lenses. The apparatus includes a projection lens and a half mirror common to a red and blue cathode ray tube. The projection lenses are arranged so that their optical axes are parallel with each other and perpendicular to the viewing screen. As a result, the projected images are displaced to the right and left of the center of the screen. This displacement is said to be correctable by outwardly and epaxially displacing the cathode ray tubes with respect to the optical axes. However, trapezoidal distortion of the hitherto misaligned, now aligned, images results. This distortion is stated to be corrected by inclining the displaced axis tubes outwardly. This inclination, in conjunction with the refractive index of the glass face plates of the cathode ray tubes, is said to correct the trapezoidal distortion.
Examples of circuit means intended to provide convergence of multiple color image projectors are to be found in the following U.S. Pat. Nos.
Seright--2,654,854
Mengle--2,989,584
Austefjord--3,943,279